1. Field of the Invention
The present invention relates to an optical fiber amplifier having constant output power for each channel and an amplifying method thereof.
2. Description of the Related Art
In an optical transmission system, an erbium doped fiber amplifier (EDFA) directly amplifies an optical signal that has been weakened during transmission without converting the optical signal into an electrical signal, thus remarkably extending the transmission distance. U.S. Pat. No. 5,768,012 to Raymond Zanoni et al. entitled Apparatus And Method For The High-Power Pumping Of Fiber Optic Amplifiers and U.S. Pat. No. 5,673,129 to Victor Mizrahi entitled WDM Optical Communication Systems With Wavelength Stabilized Optical Selectors are examples of known optical amplifiers.
Wavelength division multiplexing (WDM) optical fiber amplifiers can be generally classified into two groups; automatic gain control (AGC) optical fiber amplifiers in which the gain for each channel is maintained to be constant, and automatic level control (ALC) optical fiber amplifiers in which the output power level for each channel is maintained to be constant.
An AGC optical fiber amplifier can compensate a loss generated in a transmission path because the gain for each channel is maintained at a constant level according to a variation in the power levels of all input signals. In other words, the power level of a signal amplified by an amplifier gradually decreases while passing through the transmission path, and then restored to its original level after passing through the AGC optical fiber amplifier having the same level of the gain as that of the transmission loss. However, the performance of a general optical amplifier used as the transmission path slowly degraded with passing of time and a loss between amplifiers sharply increases. Also, a loss at a certain block of the transmission may abruptly increase due to a temporary problem. In the case of employing the AGC optical fiber amplifier in a transmission path having the above-described characteristics, the power level of a light signal gradually decreases with an increase in the transmission distance due to deterioration of the transmission path or a temporary problem thereof. Repetition of this phenomenon may considerably reduce the power level of a light signal which is input to a final input port compared to an initially set power level of the light signal. Accordingly, in the case where light signals are received in the input port with less than the minimum requirement of reception sensitivity, many en-or signals may be produced.
FIG. 1A shows a change in the signal power level on a transmission path using the AGC optical fiber amplifier, in which `100` denotes an ideal output power level and `101` denotes an actual output power level. Referring to FIG. 1A, while the gain of the AGC optical fiber amplifier is maintained at a constant value according to the transmission distance, the output power levels are not constant.
An ALC optical fiber amplifier can suppress deterioration in the performance of a transmission system due to deterioration in the performance of a transmission path or an increased loss of a certain block of the transmission path, because the output power level is maintained at a constant level while the gain for each channel varies according to a variation in the power level of an input signal.
FIG. 1B shows a change in the signal power level on a transmission path using the ALC optical fiber amplifier, in which `102` denotes an ideal output power level and `103` denotes an actual output power level. Referring to FIG. 1B, while the gain of the ALC optical fiber amplifier is varied according to the transmission distance, the output power levels are maintained at a constant level. In order to maintain the output level for each channel at a constant level, the ALC optical fiber amplifier must receive channel add/drop information from the outside.